Voice recognition relates to detecting and discriminating voice and speech sounds. Voice recognition may be used in devices which are triggered by voice or speech sounds such as a baby cry recognizer. Several methods have been proposed for voice recognition including a filter bank method and a method which uses linear prediction codes. Both of these methods have a good recognition rate but are complicated and require expensive hardware.
Another, less complicated, technique utilizes the zero crossing rate (ZCR) feature of the inputted sound signal. FIG. 1 shows a graph of an exemplary inputted sound signal. As depicted, the signal traces an undulating path above and below a zero polarity axis which axis is located at the center between the maximum and minimum signal levels of the inputted sound signal. The ZCR is simply the rate in which the path traced by the sound signal crosses the zero polarity axis (i.e., the number of zero crossings in a frame or time interval divided by the duration of that frame or time interval). Generally, ZCR techniques have a substantially lower recognition rate than the two aforementioned techniques.
Several ZCR devices have been proposed for voice recognition. R.O.C. Patent Nos. 28117 and 40308, propose systems in which sound signals both with and without voice sounds are analyzed. Thereafter, a comparison, such as a distance measurement, is performed on sound signals containing voice sounds. Such systems are complex as both voiceless and voiced sound signals must be analyzed.
U.S. Pat. Nos. 4,763,278 and 4,780,906, disclose speech recognizers which convert the ZCR of an inputted sound signal into a feature vector containing information about the ZCR and energy of the inputted sound signal. The feature vector is compared to predetermined template vectors. The recognized word may then be selected as the word having a template vector which best matches the feature vector of the inputted sound signal. A device according to these patents, however, is complex as it requires the computation of a feature vector.
U.S. Pat. No. 4,799,171, discloses a device which uses the ZCR of an inputted sound signal to determine whether the frequency of the sound signal is in a low frequency band (indicating that the sound signal contains the word "yes") or a high frequency band (indicating that the sound signal contains the word "no"). Such a device does not provide an acceptable recognition rate and is thus used only for toys.
It is an object of the present invention to overcome the disadvantages of the prior art devices.